1. Field of the Invention
The present invention relates to a method of reducing block noise as well as mosquito noise in an image, which noises are caused at the time of decoding encoded, compressed image data on a block-by-block.
2. Related Art
In order to produce a highly compressed image data, an orthogonally transformation encoding as a highly effective image data compression technique is generally employed. This technique in summary involves dividing an input image signal into blocks of such as 8×8 pixels by a blocking circuit, subjecting the divided blocks to an orthogonal transformation process by an orthogonal transformation circuit to generate a frequency component signal, subjecting the orthogonally transformed data to a linear quantization process by a quantization circuit using a predetermined quantization step width, and allocating a variable length code to the result of the quantization by a variable-length encoding circuit to generate an encoded image signal. This orthogonal transform includes such as discrete Fourier transform, Walsh-Hadamard transform, Karhunen-Loevel transform and discrete cosine transform (DCT). Of them, the DCT is most widely used.
On the other hand, in a decoding apparatus for regenerating an image signal corresponding to the input image signal from the encoded image signal generated by the above compression encoding apparatus, the encoded image signal is subjected to variable length decoding by a variable-length decoding circuit, and reverse quantization is performed by a reverse-quantization circuit using a predetermined quantization step width and the image signal is regenerate by reverse orthogonal transform.
Herein, the orthogonal transform and the quantization which are both nonreversible transform contain errors in the regenerated image signal obtained by the decoding apparatus. Particularly, quantization errors in quantization and reverse quantization deteriorate the quality of the regenerated image. The larger the quantization step width (or the larger the compression rate), the greater the number of quantization errors caused and hence the more noticeable the deterioration of the quality of the regenerated image signal. Image deteriorations inherent to such orthogonal transform include block noise and mosquito noise, the former resulting from discontinuity along the boundary of adjacent blocks which is perceived like a mosaic appearance, the latter being perceived like swarms of mosquitoes clustered around a contour of a character or a figure on the background of the image (hereinafter referred simply to “a contour”).
In order to reduce block noise, Japanese Patent Application Laid-open No. 1998-191335 (Prior Reference 1) proposes a technique that involves detecting boundaries of the blocks and filtering the pixels on the boundaries, thus correcting an image. In order to reduce mosquito noise, Japanese Patent Application Laid-open No. 1998-164576 (Prior Reference 2) proposes a technique that involves taking a difference between a target pixel and its surrounding pixels such as ±3 pixels, and applying a certain weighing coefficient thereto and adding the result to the target value when the absolute value of the difference is smaller than a threshold value.
The block noise reducing method as proposed in the Prior Reference 1 may be incapable of determining whether an object concerned is a boundary causing a great difference in pixel value between adjacent pixels due to block noise generated or only a contour of a character or a figure in image having a large difference in pixel value between adjacent pixels. If the contour in the image is erroneously determined as a boundary of the blocks and accordingly subjected to a block noise reducing process, the contour in the image may be blurred, which leads to deterioration of the image quality.
The mosquito noise reducing method as proposed in the Prior Reference 2 is effective since it is a simple filtering process and therefore takes only a short time for the processing; on the other hand, this simple filtering process is hard to properly reduce mosquito noise without deterioration of the image quality because it only involves a somewhat rough process, which may also lead to blurred contour and hence deterioration of the image quality.
As long as quantization errors are inevitably caused by quantization and reverse quantization, both block noise and mosquito noise appear in relevant regions of a regenerated image, whether they are great or not. In spite of this fact, there are no techniques, which have been found up to the present, to satisfactorily reduce both block noise and mosquito noise.
It is an object of the present invention to provide a method of reducing noise in images that is capable of properly reducing both block noise and mosquito noise without deterioration of the image quality.